1. Field of the Invention
The invention mainly relates to a fusion antigen. More particularly, the invention mainly relates to a fusion antigen used as a T-cell vaccine.
2. Description of the Related Art
The mechanism of immunization in an animal comprises humoral immunity and cell-mediated immune response.
Humoral immunity mainly relates to the production of antibodies. Antibodies can provide protection from pathogens or their toxic products by binding to them and thereby blocking their access to cells that may be infected or destroyed. Antibodies can also trigger a phagocytic cell to ingest and destroy the pathogen, such as a bacterium. The third function of antibodies is to activate a system of plasma proteins, known as a complement, that can directly destroy bacteria.
Cell-mediated immune reactions depend on direct interactions between T-lymphocytes and cells bearing the antigen that T-cells recognize. These cells recognize body cells infected with viruses, which replicate inside cells using the synthetic machinery of the cell itself. Antigens derived in the replication of a virus, however, are present on the surface of infected cells (by MHC class I), where they are recognized by cytotoxic T-cells (CD8+ T-cells) and these may then control the infection by killing the cells before the replication of a virus is complete.
Vaccines for prophylaxis of viral infections are usually live attenuated organisms with reduced pathogenicity that would stimulate protective immunity. Foreign proteins of a live virus that is used as a live attenuated vaccine, are recognized and processed in the endoplasmic reticulum (ER) lumen of antigen presenting cells (APCs) when the virus replicates to form an endogenous processing peptide. The process includes antigen modification and proper digestion. However, a live attenuated vaccine has a quite strong tendency to recover toxicity. For example, the toxicity of infectious laryngotracheitis virus (ILTV) recovers both in vaccine or attenuated strains. Besides, multiple passages of a virus should be operated; therefore, the ability to evoke an immune response is discredited. It is a time-consuming job to develop a live attenuated vaccine.
To prevent the recovery of a live attenuated vaccine, gene deficient vaccines are developed, such as Aujeszky's disease vaccines, gI negative vaccines, and PRV marker vaccines.
In another aspect, recombinant subunit vaccines and DNA vaccines are also disclosed. Viruses or bacteria of vaccina or fowlpox are used as vectors for carrying the genes of the antigens. Through recombinant DNA technology, the time for development of a good vaccine is reduced and multiple serotypes of vaccine can be achieved at the same time. Examples of such vaccines are fowlpoxvirus and Salmonella vector systems and Syntro Vet (US) gene recombinant vaccines. On the other hand, when a microorganism, especially an RNA virus, is used as a vector, the microorganism would derive a new species or a new strain. The safety of such vaccines is challenged. Besides, recombinant subunit vaccines are usually helpless in triggering a cell-mediated immune response. They are exogenous antigens, which are taken into macrophages, dendritic cells and B lymphocytes. Peptides from exogenous antigens are generated after the internalization of the antigens within APCs via fluid phase pinocytosis or membrane-bound receptors. The peptides are generated in the endosomal compartments of the APCs and sorted by empty MHC class II molecules to form peptide-MHC class II complexes based on the affinities between the MHC class II molecules and the peptides. The peptide-MHC class II complexes are then translocated to the surface of the APCs where they are recognized by CD4+ T-cells. However, small subunit proteins recognized by CD8+ T-cells cannot be used efficiently as vaccines because, once parenterally administered, they are internalized in endosomal compartments where they are likely to be either extensively degraded or fail to interact with the MHC class I pathway. Furthermore, CD4+ cells (Th cells) can both activate humoral immunity and cell-mediated immune response by Th1 and Th2 helper T-cells, respectively. Th1 and Th2 cells regulate each other for the balance of humoral immunity and cell-mediated immune response. Therefore, if only humoral antibodies will produce in all the immune responses, viral infection will be less controlled because of over sensitization of an immune system. Fortunately, it is now possible to envisage the preparation of safe T-cell vaccines able to induce protective cell-mediated immunity against all viruses (Constantin A. Bona, et al. 1998. Immunology today vol 19. 126-131).
Vaccines for virus-infecting immunological cells such as T-cell, B-cell, dendritic cell, monocyte, and macrophage still remain to be developed. Examples of such viruses are porcine reproductive and respiratory syndrome virus, Circovirus type II, and human immunodeficiency virus. Such viruses shut down the ability of recognition of foreign proteins as antigens in the antigen presenting cells. The immunological cells cannot function and carry the viruses. This kind of infection usually leads to acute damage to the animal infected. The animals that have been infected are easily infected by other pathogens. A recent report shows that cytotoxic T-cells (CTLs) are essential for controlling HIV infection. (Hanne G-S et al 2000. Journal of virology vol 74, No. 4. p. 1694-1703). It is a pity that a useful vaccine for virus-infecting immunological cells is still lacking.
In particular, porcine reproductive and respiratory syndrome virus (PRRSV) results in high losses in animal husbandry every year. The virus infects macrophages (in the alveolar and spleen), brain microglia and monocytes, and exists in the blood and organs of the infected animals. Antibodies have little effect on the virus and even stimulates mutations of the virus. In the mechanism of antibody dependent enhancement (ADE), the use of antibodies lead to more severe infections. About 50 to 80% of pigs are infected by such virus. Generally, the animals infected by the virus have no significant symptoms, but the immunity of the infected animals will be reduced. This leads to a decrease of weight gain and an increase in death rate. PRRSV is an RNA virus. Not only animals, but also ducks would be infected by PRRSV. A live attenuated vaccine against PRRSV was developed. However, mutation of the viruses in the live vaccine quite often occurs. To develop a safe vaccine is desired.